This invention relates generally to well logging methods for investigating the subsurface earth formations traversed by a borehole and, more specifically, relates to an improved method for detecting fractures and other highly permeable localized zones.
It is well known in the art of well logging to inject radioactivity tracers and to log the movement of the tracer material throughout the formations and behind the casing to provide an indication of fractures, highly permeable zones, and fluid channels behind the cement which holds the casing in place. Examples of such tracer materials are described in U.S. Pat. No. 3,600,582 to Arthur H. Youmans and assigned to the assignee of the present application. In such prior art methods utilizing radioactive tracers, the tracer material can be one of several, for example, cadmium 109. Another example of such a radioactive tracer is tritium. However, such radioactive tracers are dangerous and require special precautions and governmental permits to transport, handle and use them in the oilfield. Furthermore, such tracer materials can present obvious environmental problems.
It is also well known in the art of well logging to use gamma ray detection systems to perform spectral analysis, for example, as described in U.S. Pat. No. 3,940,610 to Charles L. Dennis et al. With a system described therein, a gamma ray detector provides a measurement of the total natural gamma radiation occurring within the borehole and three energy band selectors discriminate this measurement of the gamma ray detector into potassium, uranium, and thorium energy band signals centered about the energy levels at which potassium, uranium and thorium exhibit peak gamma radiation intensities.
It is therefore one of the primary objects of the present invention to provide a new and improved method for conducting a well logging survey which detects fractures and highly permeable localized zones in earth formations surrounding a borehole.
The objects of the invention are accomplished, generally, by first logging the well, injecting the well with a natural tracer material and then re-logging the well to follow the path of the injected fluid.